This invention relates to the reprofiling of corneal surfaces and more particularly to a novel apparatus and method for facilitating the excising of a portion of the cornea having a particularly desired thickness profile.
Refractive surgery deals with those surgical procedures which change the focusing ability of the eye. The most basic of such techniques are quite old though efforts to modify and perfect them have been continuously underway. In general, the techniques developed to date have had increasing but not total success and consistency in the treatment of nearsightedness, but all of the techniques to date have had limited success in the surgical treatment of farsightedness, astigmatism, presbyopia and irregular astigmatism.
FIG. 1 diagrammatically illustrates a front view of the cornea designating areas A and B. Also designated are meridia C and D and the optical center of the cornea, point E. Refractive surgery deals with the curvature of the cornea. Corneas that are too steep can be thought of as inducing nearsightedness and corneas that are too flat cause farsightedness. Corneas with unequal curvatures such as along path C versus path D, similar to a football, cause regular astigmatism.
In the case of nearsightedness, the cornea is too steep which is the same as saying the radius of curvature is too small. Surgery involves flattening the cornea. These surgical methods either operate in the central cornea, area A or in the peripheral cornea, area B. In farsightedness, the cornea is too flat which is the same as saying the radius of curvature is too large, and here again surgical methods exist to steepen the cornea.
Presbyopia is a problem that develops in middle age in which the eye cannot focus at near if it sees clearly at distance. Patients who are farsightedness become presbyopic earlier in life. This is not a corneal problem but one of the lens inside the eye, however, it is theoretically possible to reshape the cornea surgically to produce a bifocal shape for both distance and near vision. Irregular astigmatism is a situation where curvatures along the cornea, as for example along meridia C and D are irregular, like a football with the air out of it. To
date the most experience has been with nearsightedness correction as compared to farsightedness. Little data exists with respect to the treatment of presbyopia by making the cornea bifocal and no data exists, to our knowledge, to treat irregular astigmatism with corneal surgery.
Radial keratectomy is a relaxing procedure in which 90-95% depth radial cuts are made in the cornea in area B of FIG. 1, sparing a central optical zone of varying diameter. Drawbacks of this procedure include a limited range of correction, fluctuating vision, glare, and continued relaxation, leading to farsightedness, years after the procedure. Transverse incisions into the cornea in area B can be used to treat astigmatism.
Keratomileusis is also an old procedure which involves removing the anterior third of the cornea, freezing it, then placing it on a lathe. Layers of the cornea from the posterior surface of area A are then lathed away after which the reconfigured section is sutured back onto the cornea. This technique is useful for very high degrees of myopia and hyperopia.
Probably the most standard work on the subject of Keratomileusis is a Spanish language text "Queratomileusis-Querotofaquia"by Jose Barrequar who is considered one of the fathers of keratomileusis. Dr. Barrequar flatly states (pages 77-88) that attempts to depress the center of the cornea during keratomileusis will not work, and that the resulting excised section will merely be parallel to the original corneal section
A more recent technique is excimer laser sculpting of the cornea. Here the corneal epithelium is scraped off to expose Bowman's membrane which covers the stroma. The Excimer laser then removes layers of cornea, including Bowman's membrane, in such a manner as to leave the curvature of the surface of the stroma changed, such that when the epithelium covers the stroma, the front curvature is different, thus correcting vision. This procedure suffers from slow, unpredictable healing at the epithelial-stromal interface caused by the absence of Bowman's membrane.
Another approach in excimer laser techniques of this type involves the use of an erodible mask such as is disclosed and claimed in U.S. Pat. No. 4,856,513 to Muller. In these techniques, the erodible material is placed between the Excimer laser beam and Bowman's membrane. As the laser is applied, it erodes through the mask, so that the laser quickly penetrates the mask at that point where it is thinnest, and the cornea under the thinner parts of the mask receives more exposure to the laser beam than the thicker portions. This, in turn, results in a different energy distribution on the cornea which, in turn, results in different cutting in different places and thus the refractive change. These techniques with the excimer laser are limited to nearsightedness and astigmatism particularly because of healing of the epithelium. The treatment of hyperopia has not proven successful because the epithelium tends to thicken and fill in attempts to steepen the cornea by excising less tissue centrally and more peripherally.
The most recent development is the use of a modified keratomileusis procedure, which may generally be described as keratomileusis in situ. This technique involves removing the anterior third of the cornea, after which a second cut of varying thickness, in an exact optical zone in the range of from about 3.5 to about 5.0 millimeters, is produced in the corneal bed, after which the piece which was originally removed is replaced. The missing portion removed from the corneal bed by the second cut results in a flattening of the corneal curvature and, thus, a correction of nearsightedness.
Keratomileusis in situ is described by Dr. Luis Ruiz whose apparatus and techniques are more particularly described in European Patent Application No. 90201166.7 (which has been published as Publication No. WO/0442156A1). As used herein, the terms "Ruiz Method" and "Ruiz Apparatus" shall be understood to mean the method and apparatus disclosed by Dr. Ruiz's European Patent Application No. 90201166.7, the drawings and specification of which are specifically incorporated herein by reference. Dr. Ruiz describes his apparatus as being made up of three main parts: a motor and transmission shaft assembly, a shaper head assembly, and a retaining ring assembly, the overall device being specifically designed to automatically perform corneal resections.
By causing the shaper head assembly to move automatically and smoothly at a constant speed across the retaining ring assembly which holds the eye in position for resection, precise uniform reproducible cuts are possible. In using the apparatus of Dr. Ruiz, the device is fitted over the cornea with the underside of the shaper head assembly depressing the surface of the cornea inwardly to an essentially flat configuration across a defined optical zone, and the keratome makes a first cut of about 160 microns. This cut is of essentially uniform thickness and is essentially parallel to the flattened corneal surface. The excised portion of the cornea is saved, the shaper head assembly is refitted over the eye, again in engagement with and flattening the surface of the cornea and a second smaller portion of exact diameter is removed from the corneal bed. The first excised section is then replaced over the cornea, and the second excised section is discarded.
The Ruiz Method is illustrated in FIG. 2a-i where a cornea, generally designated 1, has an anterior surface 2 comprised of layer of epithelium 21, upon Bowman's membrane 211. Numeral 3 designates what will be referred to as the corneal bed or `stroma` through which the plane of resection will be located. In FIG. 2a the cornea is shown with reference to the underside surface 8 of the Ruiz shaper assembly, prior to engagement of the apparatus over the eye. FIG. 2b illustrates the elements of FIG. 2a after engagement showing corneal surface 2 flattened by underside surface 8 and showing dotted line 9 which is the cutting path of the keratome. FIG. 2c shows the cornea after the first keratome cut and FIG. 2d shows the portion of the cornea, designated 4, which was excised. FIG. 2e shows surface 8 reengaged for a second cut by the keratome along dotted line 9a in corneal bed 3. In general, the second cut is not as deep nor as large in diameter as the first cut. FIG. 2f shows the corneal surface after the second excising and FIG. 2g shows cornea 1 after the second excising and subsequent reattachment of first excised portion 4. Dotted line 9b designates the cut made in stroma to produce flattening of surface 2 as indicated. It should be noted that the flattening produced is extremely dependent upon the depth and importantly, the diameter of the resected piece. If the resected piece is too large, little effect is achieved because most of the resection is parallel to the original surface 2 whereas at smaller zones of resection, the resection is more lenticular and this results in flattening of the cornea.
While Dr. Ruiz suggests the potential use of his techniques for the correction of farsightedness as well as nearsightedness, he suggests doing so in a manner radically different from that in which it is normally employed in the correction of nearsightedness. This is so because the method of Dr. Ruiz cannot selectively remove tissue located more peripherally than centrally to steepen the bed. FIG. 2h shows the hypothetical cut, 9c, that Dr. Ruiz cannot produce to steepen the bed and thus steepen the corneal surface 2 once 4 is reapplied.
In attempting to correct farsightedness, Dr Ruiz makes a very deep cut with the keratome (about 75%), remove and then reapplies the excised portion. FIG. 2i shows the cut as 9d with back surface of the cornea 10. The first cut causes the remaining 25% of the cornea to bow out, in turn resulting in a steepening, the amount of steepening being determined by the diameter of the cut and the amount of bowing out which actually takes place. This technique is potentially dangerous and depends upon uncontrolled stretching of the cornea to achieve the desired steepening.